Information Page: Sinking Deltas
Sinking Deltas Explained
|Domain:||terrestrial, hydrology, coastal, environmental controls|
|Model name:||Animation model name|
Grade level: High (9-12), Under graduate (13-16), Graduate / Professional
Statement: Major world deltas are sinking relative to sea level
Abstract: CU Science update featured a short movie on 'Sinking Deltas'. There is a widespread need to assess the vulnerability of the world’s population living in low-lying deltas to flooding, whether from intense rainfall, rivers or from hurricane-induced storm surges. High-resolution NASA SRTM topography data and MODIS satellite data along with georeferenced historical map analysis allows quantification of the extent of low-lying delta areas and the role of humans in contributing to their vulnerability. Thirty-three major deltas collectively include ~26,008 km2 of area below local mean sea level and ~96,000 km2 of vulnerable area below 2 m a.s.l. This vulnerable area may increase 50% under projected 21st Century eustatic sea level rise. Analysis of river sediment load data and of topographical changes show that these densely populated, intensively farmed landforms, that often host key economic structures, have been destabilized by human-induced accelerated sediment compaction due to water, oil and gas mining, and by reductions of incoming sediment from upstream dams and reservoirs and floodplain engineering.
Close to 0.5 billion people live on, or near, world deltas, inclusively in many mega-cities. Ten countries (China, India, Bangladesh, Vietnam, Indonesia, Japan, Egypt, USA, Thailand, and the Philippines) account for 73% of the people that live in the world’s coastal zone, defined as within 10 m of mean sea level. 20th-century catchment developments and population and economic growth within subsiding deltas have placed these environments and their populations under a growing risk of coastal flooding, wetland loss, shoreline retreat, and loss of infrastructure.
To assess vulnerability of deltaic lowlands one has to look at a delta as a balance. A delta’s surface elevation above mean sea level can experience a vertical change relative to local mean sea level, ΔRSL. It is controlled by a summation of 5 factors:
(1) ΔRSL = A - ΔE - Cn - CA ± M
A delta’s Aggradation Rate (A) is determined from the volume of sediment delivered to and retained on the subaerial delta surface as new sedimentary layers due to flooding. A typically varies from 1 to 50 mm/y in deltas worldwide (See Table 1 in Syvitski et al., 2009). Most river floods bring large amounts of sediment to a delta’s surface. Reducing the number of distributary channels along with artificial levees can prohibit river flooding onto the delta plain. Flooding from ocean surges may still contribute additional turbid water. ΔE is the Eustatic Sea Level Rate determined from changes to the volume of the global ocean over time, as influenced by fluctuations in the storage of terrestrial water (e.g. glaciers, ice sheets, groundwater, lakes, and reservoirs), and fluctuations in ocean water expansion due to water temperature changes. Presently ΔE is positive and contributes ≈1.8 to 3 mm/y (IPCC, 2007) under the anthropogenic influence of global warming. The IPCC projects that sea level will rise another 21 to 71 cm by 2070, with a best estimate of 44 cm averaged globally; it is becoming increasingly clear that the major ice sheets might contribute even more water over this period. Natural Compaction (Cn), or Accelerated Compaction (CA) reduce the volume of deltaic deposits. Cn involves natural changes in the void space within sedimentary layers (e.g. dewatering, grain-packing realignment, and organic matter oxidation), and is typically ≤3 mm/y. CA is the anthropogenic contribution to volume change as a consequence of subsurface mining (oil, gas or groundwater), human-influenced soil drainage and accelerated oxidation. CA can exceed Cn by an order of magnitude. M is the typically downward vertical movement of the land surface as influenced by the redistribution of earth masses (e.g. sea level fluctuations, growth of delta deposits, growth or shrinkage of nearby ice masses, tectonics, and deep-seated thermal subsidence). M is highly variable spatially but rates are typically between 0 and -5 mm/y.
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